This invention relates to electrodes for use in alkaline storage batteries and more particularly to cathode or positive electrodes which contain nickel hydroxide as the active material.
Nickel electrodes of this type are commonly employed in batteries having anodes containing Cd, Zn, H.sub.2 and other electrodes also contain graphite as a conductive diluent to impart electronic conductivity to the active material. The conductive graphite diluent is used in place of the more conventional sintered nickel plaque structure and results in a lighter weight and less costly battery construction.
However, there is one significant drawback to using graphite diluent in nickel electrodes. The electrodes exhibit lower performance at high rates of discharge as compared with the more conventional electrodes using sintered nickel plaques. Lower performance at rates of discharge higher than, for example, the I-hour rate (i.e. 50 MA/cm.sup.2), are the result of the higher internal resistance exhibited by the electrodes. Higher internal resistance is caused, in part, by the poorer conductivity of the Ni(OH).sub.2 to graphite particle contact, especially after the electrode is more than 50% discharged. This poorer conductivity manifests itself by increasing electrode polarization and lowering active material utilization particularly during high rate discharges.
There have been attempts to improve the high rate performance of nickel electrodes employing graphite diluents. One attempt involved the admixing or coating of a conductive metallic powder (e.g. Ni, Ag, Mn, etc.) on the graphite particles to improve their overall electrode conductivity. However, it has been found that these metallic coatings form oxide layers which in time also result in a lower high rate discharge performance for the electrode.
Another attempt at improving these electrodes involved trying to improve the conductivity of the Ni(OH).sub.2 active material by doping the material with Li.sup.+, Al.sup.+3, Mg.sup.+2 or Mn.sup.+2 ions. This approach, however, also has resulted only in improvements of a transitory nature.
It is, therefore, an object of the present invention to provide an improved nickel electrode which exhibits lower polarization and improved active material utilization at high rates of discharge.
It is a further object of the present invention to provide a nickel electrode which contains a graphite diluent and which exhibits lower polarization and improved active material utilization for high rates of discharge.
It is yet a further object of the present invention to provide a nickel electrode which exhibits the above properties after repeated charge-discharge cycling.
It is still a further object of the present invention to provide a method for making active material for the aforementioned electrodes.